Backport of docs: add a missing letter to the upgrade guide into release/1.10.x

[hc-github-team-secure-vault-core]

Backport

This PR is auto-generated from #17554 to be assessed for backporting due to the inclusion of the label backport/1.10.x.

WARNING automatic cherry-pick of commits failed. Commits will require human attention.

merge conflict error: POST https://api.github.com/repos/hashicorp/vault/merges: 409 Merge conflict []

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• 8b8c077

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[schavis]

Closing failed auto-backport PR that is more than 1 month old